Available spectrum is becoming increasingly burdened by ever greater demand for video information channels. Traditional airwave spectral space has been crowded for many years, and burgeoning video programming for such applications as home cable, teleconferencing, picture phones, and computer video transmission has now crowded conductive and optical cables, phone lines, and sattelite communication channels. The desirability of techniques for increasing the amount of video information that can be sent over these transmission media is evident. Also, as more video information is stored, it is desirable to develop techniques that increase the amount of video that can be stored in a given storage size.
As high definition television (HDTV) becomes more prevalent, improved systems are needed for transmission and reception of the additional information required for presenting HDTV images. Any new service which provides higher definition television than is conventionally broadcast (i.e., more elements per line and lines per frame, and thus a wider bandwidth necessary for transmission) should serve existing home television receivers with essentially all the picture attributes and quality of which the receivers are capable. Also, receivers designed for new (high definition) service, should be capable of operating using the pre-existing transmissions and derive from them a result not inferior to that provided by pre-existing receivers.
A variety of HDTV schemes have been proposed. In U.S. Pat. Nos. 4,517,597, 4,628,344, 4,652,909 and 4,701,783, and 4,800,426, assigned to the same assignee as the present application, as well as in the publication "HDTV Compatible Transmission System", W. E. Glenn, National Association of Broadcasters, April, 1986, there is disclosed an HDTV system that utilizes an augmentation approach which permits compatible transmission of HDTV. A separate auxiliary or "augmentation" channel is used to send picture detail information that augments conventionally received television information to obtain high definition performance. The disclosed techniques also have application to video bandwidth compression and to reducing video storage capacity.
As described in the referenced patents and publication, an electronic video signal (e.g. a television signal) can be encoded at reduced bandwidth by lowering the frame refresh rate of the high spatial frequency components, while maintaining the frame refresh rate of at least a portion of the low spatial frequency components at the standard rate. If done in a specified manner this will not cause substantial degradation in the ultimately displayed image, since human vision cannot perceive changes in high spatial resolution information at as fast a rate as it can perceive changes in low spatial resolution information. Accordingly, as has been previously set forth, an electronic video encoding and decoding system can be devised which takes advantage of this, and other, characteristics of human vision by encoding higher spatial resolution video components to be at a temporal information rate which approximately corresponds to the highest rate actually perceived by human vision for such components; thereby eliminating the need to encode these components at a higher rate, which inherently wastes bandwidth. Also, as shown in referenced patent and publication, the low spatial resolution information can be generated in a form which is compatible with standard television video, for example NTSC video used in the U.S. It has also been recognized that a number of frequency components can be transmitted at specified rates [see e.g. W. F. Schreiber et al., Reliable EDTV/HDTV Transmission In Low Quality Analog Channels, SMPTE Journal, July 1989, and the abovereferenced patents of the present assignee], with components selected according to degree of motion in order to have higher spatial resolution in scenes with little motion and higher temporal resolution in scenes with a great deal of motion.
FIG. 1 illustrates a compatible high definition television transmission and receiving system of the general type described in the above-referenced patents and publication. A transmitter 200 includes NTSC processing circuitry 210 which processes television signals from a source such as a television camera system (not shown) or a video recording system (not shown). The circuitry 210 is coupled to transmitting circuitry 215, which typically includes modulation circuitry and other suitable circuitry for producing a signal to be transmitted over a standard NTSC channel. The television signals from the television camera system or video recorder (which is assumed to have a high definition video capability) are also processed by high definition television (HDTV) processing circuitry 260 which produces detail signals that can be utilized to enhance conventional television signals to obtain HDTV signals, as described in the abovereferenced patents and publication. (As further described in the referenced U.S. Pat. No. 4,652,909, the detail signal can be obtained from a separate camera.) The detail signals are coupled to further circuitry 275, which transmits the detail signal over a second (auxiliary) channel that is typically not adjacent to the (main) NTSC channel used for transmission of the standard portion of the television information. The NTSC signal is received by receivers such as receiver 310 which has only a capability of producing a television picture at substantially conventional resolution e.g. conventional display 315. Receivers such as receiver 360, which have a capability for receiving, processing, and displaying high definition television signals, receive both the main channel carrying the NTSC signal and the auxiliary channel carrying the detail signals to be used for augmentation of the NTSC video signal so as to produce a high definition television signal for display on an HDTV display 365.
In copending U.S. patent application Ser. No. 502,519 of R. Dhein filed Mar. 30, 1990, and assigned to the same assignee as the present invention, there is disclosed a method and apparatus for encoding (for storage and/or transmission) and decoding video to achieve bandwidth compression. In a disclosed embodiment of that Application, information content is reduced by eliminating high diagonal frequency components of video, spectral transforms are utilized, and frequency bands of detail signal are selected for each tile of the picture depending on the motion status of the particular tile.
As further background to the invention, reference can be made to the following U.S. Patents which relate to compression, transmission and/or other processing of video signals and/or still picture information:
______________________________________ U.S. Pat. No. 4,196,448 4,210,931 4,224,678 4,302,775 4,394,774 4,541,012 4,605,952 4,630,099 4,661,862 4,672,425 4,675,733 4,675,750 4,729,012 4,774,561 4,780,761 4,791,598 4,807,029 4,821,119 4,845,562 4,851,906 4,870,489 4,873,573 ______________________________________
The FCC recently announced that it prefers planned terrestrial HDTV transmission in the U.S. to be broadcast using a simulcast format: i.e., with the same program content sent simultaneously both a conventional television channel and a separate HDTV channel. It has been anticipated that, in time, television viewers will replace standard NTSC receivers with high definition sets, thereby allowing the present NTSC channels to eventually be reassigned for other application. In order for this concept to work, however, viewers must be motivated to purchase receivers designed to accept this new format. Even when wide-screen HDTV becomes available, a significant demand will always exist for smaller-screen receivers. The image quality of small screen-size television receivers is generally not limited by transmission considerations, but by human visual acuity. The optimum viewing distance for popular 19-20" conventional receiver, for example, is between six and seven feet. A similar screen-size HDTV receiver has an optimum viewing distance of about three feet; clearly impractical in most viewing situations. The goal of abandoning the conventional NTSC channels in the foreseeable future may be impractical because there will always be a consumer demand for inexpensive smaller screen television sets.
It is among the objects of the present invention to provide improvements in encoding and decoding of video information which addresses the described problems and limitations of the prior art, achieves substantial bandwidth savings, increases the efficiency of video transmission and storage, and provides a capability for higher definition television transmission in the bandwidth of a single conventional television channel. It is also among the objects of the present invention to provide a technique whereby two video signals, representative of different images can be transmitted using only the bandwidth generally allocated to a single video signal, with little or no perceived degradation of image quality. It is also among the objects of the present invention to provide a method for broadcasting video signals with improved interference immunity.